Tube with tunable coaxial resonator



Patented Oct. 28, 1947 TUBE WITH TUNABLE COAXIAL RESONATOR John J. Guarrera, Astoria, N. Y., assignor, by mesne assignments, to the Government of the United States of America, as represented by the Secretary of War Application August 1, 1945, Serial No. 608,302

3 Claims. `l

This invention relates to electrical circuits and more particularly to oscillator circuits involving coaxial lines as resonant circuits.

According to conventional practice transmission lines rather than inductive and capacitive elements are often used as the resonant circuits of oscillators. In general an oscillator has two or more resonant circuits, one or more of which may be varied to change the frequency of oscillation of the oscillator. It is, in all cases, desired to have the impedance of the resonant circuit as seen by the loscillator tube at the highest possible value. In cases where transmission lines are used the line is open or short-circuited at the frequency of oscillations at the end remote from the oscillator tube and its length is a half or quarter wavelength respectively, depending upon the type of termination.

In the conventional oscillator the termination used has been a short circuit produced in general by a second section of line which was opencircuited. As is Well understood in the art, however, it is impossible with such a termination to obtain a perfect short circuit. Furthermore, since the termination involves a second open-cir- Vcuited line, some energy will be lost.

The object-of the present invention is to provide an improved termination for transmission lines used as resonant circuits in which the disadvantages set forth above are overcome.

In accordance with the present invention there are provided two coaxial conductors which form -a resonant circuit for Van oscillator. There is also provided a plunger inserted between the two coaxial conductors. stance, such as mica, is inserted between the plunger and one of the two conductors in such a manner as to present an open circuit for direct current between `plunger and conductor and Vat the same time afford a relatively large capacitance between the plunger and conductor. The plunger makes direct contact with the remaining conductor to afford a D.C. path.

' For a" better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken A piece of dielectric sub- 1 in connection with the accompanying drawings in which:

Fig. 1 shows an embodiment of the invention in a lighthouse triode oscillator circuit;

Fig. 2 is a line drawing .of .the conventional transmission line termination; and

Figs- 3 and 4 are line drawings of the type of transmission line termination-used in this invention.

Referring now more particularly to Fig. l of the drawing, there is shown an embodiment of vthis invention in a. lighthouse triode oscillator circuit. The circuit comprises a tube, shown in partial cross-section, I I having a cylindrical cathode terminal I2, an annular grid terminal I3 .and a cap shaped plate terminal 14. Cathode terminal I2 is capacitatively coupled to a cathodeelectrode I2. Grid terminal I3 is connected to .a grid electrode I3', and plate terminal i4 is vconnected to a plate electrode I4. The tube H shown in this embodiment is a tube which has been especially designed for usein the ultra-high frequency region and in conjunction with three coaxial lines .as shown. This tubeis knownin the art as the lighthouse tube because of its physical appearance. A tubular conductor I5 connects to the plate terminal I4. A second .tubular conductor I5, which `is coaxial with conductor I5. connects to the grid terminal I3. A third tubular conductor 2l, also coaxial with conductor I5, connects to the cathode terminal I2. A dielectric substance 22, Vpreferably mica, separates a termination '23 fromithe conductor I5. The termination 23 may be in the form of a plunger which makes a sliding contact with the conductor 2 I.

The operation of the termination toward which the invention is directed is deferred until the line drawings of Figs. 3 vand4 `are discussed. The oscillator circuit here shown is essentially what is known as atuned-plate,tunedgrid,tunedcath ode circuit. The conductors I5 and 2| form a resonant circuit between the plate I4 and cathode I2 of the tube II. This invention is concerned primarily with the termination 23 which connects the conductors I5 and 2|. The conductors I5 and I6 form a resonant circuit between the plate I4 and grid I3 of the Vtube II. The conductors I6 and 2| form a resonant circuit between the grid I3 and cathode I2 of the tube II. The operation of this circuit as an oscillator is well known in the art.

Referring now particularly to Fig. 2 ofthe drawing, there is shown an Vequivalentline drawing of the conventional termination. In 'the drawing, line 24 represents the outer conductor 2| of Fig. l. The line 25 represents the inner conductor I5 of Fig. l. The plunger 25 is a part of the termination of lines 24 and 25. YIt should be understood that this gure represents a section through only One-half of the structure, the other half (not shown in Fig. 2) being disposed on the other -side of the dot-dash'line which represents the longitudinal axis of the variousconductors. t

It is desired that theimpedance seen .at `3l be at a maximum at the resonant frequency. rIhe termination which includes the plunger 26 is normally made to approach a short circuit. The required distance 32 from the location marked 3i to the plunger 26 will-be an odd number yof quarter wavelengths, The termination of lines 24 rand 215 comprises the plunger 26 in series with the im- Vpcdances seen at 33 and 34, the junctions of 3 plunger 26 with lines 24 and 25, respectively. The impedance of the plunger 26 is zero, and it is obvious that the impedances at 33 and 34 must be at a minimum in order to aioz'd the desired short circuit. One of the impedances 33 or 34 is generally zero because sliding contact is made between the plunger and. one conductor. However,

the other impedance cannot be zero because of` the requirement that D. C. insulation be provided. The impedance at 34 will be taken to be zero. It can be shown that the impedance `at 33 is approximated by the relation:

Z Zeg In the above expression Zas is the impedance seen vat 33, Zc is the characteristic impedance of the lineformed by line 24 and line 36 of the plunger n 26, and Zas is the impedance which terminates this section of line. The length of this line is designated 4I and is normally one-fourth wavelength. It ycan be seen from the expression above that if the impedance at 33 is to be zero at the oscillator frequency, either ZC must be zero, a physical impossibility, or Zas must be infinite. Since the impedance at 35 is actually another transmission line, it cannot be made infinite.

The characteristic impedance, Zc, for a coaxial line is given by the following expression:

138 a b l 10310 In the above expression K is the dielectric constant of the medium separating the two conductors 24 and 36, a is the outside radius of the inner conductor 36 and b is the distance by whi-ch the two conductors 24 and 36 are separated.

It will be noted that increasing K from its value of unity for air will decrease ZC. We also note can be made to approach unity by increasing a or by decreasing b.

In accordance with the above considerations reference is now made more particularly to Figs. 3 and 4 in which there is shown an improved termination, In this instance the lines 24 and 25 are vterminated by a plunger 42. In Fig. 3 the plunger 42 connects to line 24 by a short circuit indicated by 43 and is separated by a dielectric substance 44 from line 25. The dotted portion 45 indicates that the substance 44 might equally well lie in that position. In Fig. 4 the positions of the short circuit 43 and the substance 44 are reversed. Although the line length 46 in the region of the dielectric is not critical, it is preferably onequarter wavelength.

In the termination of Fig. 3 maximum advantage is taken of reducing the distance b and of increasing the dielectric constant K. By the changes thus made Ze is sufficiently reduced so that the impedance seen at 41 is very low with the result that the impedance at 48 is very high. In the termination shown in Fig. 4 maximum advantage is taken of reducing b, increasing a andv increasing K. In both Figs. 3 and 4 the plunger 42 may be securely bound tc the conductor from which it is separated by the dielectric substance 44. By this method the spacing b may be decreased still further with a subsequent increase in the operating efficiency of the oscillator. In such cases it is necessary that line 25 cr line 24, whichever the case may be, move with the plunger if tuning of the oscillator of which this resonant circuit is a part is to be accomplished.

While there have been described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modicaticns may be made therein without departing from the invention.

I claim:

l. An electronic oscillator comprising an electron tube having grid, plate and cathode electrodes; iirst, second, and third coaxial cylindrical conductors, said rst and second conductors forming a resonant circuit betweensaid plate and grid electrodes, said second and third cone ductors forming a resonant circuit between said grid and cathode electrodes, said first and third conductors forming a resonant circuit between said plate and cathode electrodes; plunger means having direct contact with said third conductor; and insulating means having a high dielectric constant inserted between said plunger means and said rst conductor.

2. An electronic "oscillator comprising an electron tube having rst, second, and third electrodes; iii-st, second, and third coaxial cylindrical conductors, said first and second conductors forming a resonant circuit between said iirst and second electrodes, said second and third conductors forming a resonant circuit between said second and third electrodes, said rst and third conductors forming a resonant circuit between said rst and third electrodes; plunger means having direct contact with one of said iirst and third conductors; and insulating means having a high dielectric constant inserted between said plunger means and the remaining of said first and third conductors. Y

3. An electronic oscillator comprising a tube having rst, second, and third electrodes; iirst, second, and third coaxial cylindrical conductors, said iirst and second conductors forming a resonant circuit between said first and second electrodes, said second and third conductors forming a resonant circuit between said second and third electrodes, said inst and third conductors forming a resonant circuit between said first and third electrodes; and a termination member including a first conductive cylinder in sliding contact with said third conductor, a second conductive cylinder in concentric relation and in proximity to said rst conductor, and insulating means having a high dielectric constant inserted between said second conductive cylinder and said rst conductor.

JOHN J. GUARRERA.

REFERENCES CITED The following references vare of record in the file of this patent:

UNITED STATES PATENTS Allerding June 20, 1944 

